1. Field of the Invention
This invention relates to power couplings employing electromagnetic near fields, and in particular those using inductive or capacitive couplings to transmit electrical power to a load without electrical or mechanical contact.
2. Description of Related Art
Inductive or capacitive power couplings (herein referred to as near field power couplings) transfer power by means of electromagnetic near fields generated between capacitive plates or inductive windings. Since electromagnetic fields can travel through a variety of mediums (particularly air), electrical contacts are not required to couple power to a load. Contactless near field power couplings eliminate the friction, arcing, wear, mechanical stress, and inaccessibility associated with electrical contacts. They are particularly advantageous in powering mobile vehicles. In these applications, batteries or other energy storage devices can be eliminated while maintaining considerable mobility.
In the present invention, power is coupled by phased electromagnetic near fields. In general, electromagnetic fields are comprised of induction, electric, and radiation field components. Energy contained in the induction and electric fields does not radiate but rather is stored and returned to the circuit during each cycle. Within approximately one sixth of a wavelength from an electromagnetic field generating element, the induction and electric field components dominate. Thus, electromagnetic fields dominated by induction and electric field components are termed near fields. The energy in the near field is substantially higher than in the corresponding radiation or far field. Most of this energy is not radiated but rather is stored and returned to the circuit during each cycle.
Numerous applications for near field power couplings exist in the prior art. These include those for providing power to vehicles, such as in U.S. Pat. No. 4,800,328 issued to Bolger on Jun. 6, 1989, and U.S. Pat. No. 4,836,344 issued to Bolger et al. on Jan. 24, 1989.
Lacking in the prior art is a means for providing continuous power and control to a load as it is positioned at various locations (positional independence). This is a significant disadvantage in applications requiring a high degree of mobility. For example, a robot might be programmed to move to any coordinate in a plane over a planar surface. It would be desirable to provide near field power and control coupling to the robot at any of those coordinates. However, prior art near field applications are limited by discontinuities or dead spots in the electromagnetic field across a planar surface.
In the case of the roadway power system described in U.S. Pat. No. 4,836,344, the power receiver is limited to the area directly over each inductive module. If multiple inductive modules are activated at once, the magnetic fields from adjacent modules tend to cancel at midpoints between modules, creating dead zones in which no power is provided. These dead zones in this and in other prior art result from phase cancellation in the generated electromagnetic fields when driving two inputs to a power coupling receiver. In these circumstances, the effective result is that the two inputs to the receiver are driven in phase.